Power systems such as electrical power distribution or transmission systems are used for supplying, transmitting and using electric power. High Voltage Direct Current (HVDC) power transmission are becoming more prevalent due to increasing need for power transmission with low transmission loss and flexible interconnection possibilities.
Power systems such as electrical power transmission systems generally include a protection system for protecting, monitoring and controlling the operation of electrical devices in the power system. Such protection systems may for example be able to detect short circuits, overcurrents and overvoltages in power lines, transformers and/or other parts or components of the power system. The protection systems can include protection equipment such as circuit breakers for isolating any possible faults for example occurring in power transmission and distribution lines by opening or tripping the circuit breakers. After the fault has been cleared, e.g. by performing repairs and/or maintenance on the component in which the fault has been detected, the power flow can be restored by closing the circuit breakers.
Moreover the protection system can be arranged to, upon detection of a fault in a particular electrical device, isolate the faulty electrical device by bypassing the electrical device, using a bypass switch. The bypass switch then provides a conductive path to bypass the electrical device until the electrical device is repaired or replaced.
An HVDC converter station converts high voltage direct current (DC) to alternating current (AC) or vice versa. An HVDC converter station may comprise a plurality of elements such as a converter device (or a plurality of converters devices connected in series or in parallel), an AC switchgear, transformers, capacitors, filters, a DC switchgear and/or other auxiliary elements. Converter devices may comprise a plurality of solid-state based devices such as semiconductor devices and may be categorized as line-commutated converters, using e.g. thyristors as switches, or voltage source converters, using transistors such as insulated gate bipolar transistors (IGBTs) as switches. A plurality of solid-state semiconductor devices such as thyristors or IGBTs may be connected together, for instance in series, to form a building block, or cell, of an HVDC converter, which may also be referred to as an HVDC converter valve.
According to one example, a plurality of solid-state semiconductor devices such as thyristors or IGBTs may be connected in series in a cell of an HVDC converter. During normal operation of e.g. an HVDC power transmission system or an HVDC grid including the HVDC converter, the solid-state semiconductor devices in the HVDC converter may at times be in a conducting mode in which they are conducting current and at other times be in a blocking mode, in order to attain a desired (e.g. sinusoidal) waveform of the current. This may expose the solid-state semiconductor devices to continuous current stresses, which, especially in HVDC applications, may be of significant magnitude. If any one of the solid-state semiconductor devices fails, the current through the HVDC converter can be interrupted, and repairs and/or replacement of any failed solid-state semiconductor device might then become necessary in order to put the HVDC converter back into operation. In an HVDC converter station based on voltage source converters there may be DC capacitors, or DC capacitor banks, which act as voltage sources and which are connected to, for instance in parallel, one or several solid-state semiconductor devices such as IGBTs included in a cell of an HVDC converter.
As described above, upon detection of a fault in a particular electrical device, the faulty electrical device can be isolated by bypassing the electrical device, using a bypass switch. Such fault operation can be applied for faulty semiconductors and/or capacitor banks. However, due to the high voltages involved, arcing occurs during the switching, which deteriorates the contacts of the bypass switch, resulting in losses and/or an unstable bypass state.
It is desired to provide a bypass switch which reduces the ill-effects due to arcing.